Quantitative Evaluation of ^<67>Ga-Planar Images Acquired with Different Combinations of Photo-Peaks Using Image Frequency Characteristics

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  • 異なるphotopeakを用いて収集した^<67>Gaプレイナ画像の周波数特性による定量評価
  • コトナル photopeak オ モチイテ シュウシュウシタ 67Ga プレイナ ガゾウ ノ シュウハスウ トクセイ ニ ヨル テイリョウ ヒョウカ

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The purpose of this study was to analyze quantitatively the influence of acquisition of various combinations of photo-peaks on gallium-67 planar images using frequency analysis. A liver phantom provided by the International Atomic Energy Agency was imaged using a GCA-901WB (Toshiba, Tokyo) ; 37MBq of gallium-67 was injected into the phantom, and its planar images were acquired with each of three photo-peaks and the following combinations ; one each of 93, 185 and 300keV peaks, two of the three peaks, i.e. 93 and 185, 93 and 300, and 185 and 300 keV peaks, and all of the three peaks. Image contrast was varied with and without scatter material. Without scatter material, the 93keV-peak image had the highest contrast, and the contrast degraded when adding a higher energy photo-peak or photo-peaks. With scatter material, the image contrast decreased when the scatter material thickness increased, and the contrast degradation of the 93keV-peak image was particularly remarkable. Radius distribution function (Pr(n)) was used for quantitative frequency analysis. Images were collected up to 500 kcounts, and tough water phantoms (thicknesses: 5, 10, 20cm) were used as scatter material. The highest Pr(n) was obtained with the 93keV-peak without scatter material ; however, with a 20cm-thick tough water phantom, the image with three peaks had the highest Pr(n). According to the increase in tough water phantom thickness, the intensity of Pr(n) for the 93keV-peak in the range from 0.1 to 0.3 [cycles/cm] decreased by 20% of that without the tough water plate. When the thickness of the scatter material was over 10cm, 93keV-photons from the deep portion of the phantom were not detected properly. The optimal energy photo-peaks were three peaks for a single-head camera and the combination of 93 and 185keV peaks for a dual-head camera.

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